This invention relates to a phenol derivative or a salt thereof, which can be utilized as an agent for adjusting sensitivity at the time of measurement in a method for a colorimetric analysis of metal ions by use of a chelating reagent; and a process for using said phenol derivative or salt thereof as an agent for adjusting sensitivity for a colorimetric analysis of metal ions.
Methods for a colorimetric analysis of metal ions by use of a chelating reagent capable of forming a chelate with metal ions to develop a color have a relatively high precision and require only easy operations. Therefore, they are widely utilized in the fields of water quality analysis, clinical chemical analysis, etc.
On the other hand, the chelating reagent used for such a purpose is required to have a high specificity, a large chelate formation constant, etc. Accordingly, usable chelating reagents are limited as a matter of course, depending on the kind of metal ion. For example, for analysis for calcium, there are used chelating reagent such as phthalein complexone (OCPC), Methylxylenol Blue (MXB), Methylthymol Blue (MTB), etc. For analysis for magnesium, there are used chelating reagents such as Xylylazo Violet (XB-I), etc.
However, the molecular extinction coefficient (.epsilon.) of a chelate formed by the reaction of any of the above-exemplified chelating reagents with metal ions (hereinafter abbreviated as "color forming sensitivity") has a relatively high value. Therefore, in the case of carrying out measurement for a sample containing a high concentration of metal ions to be measured, for example, the case of measuring calcium or magnesium in serum, the problem 1 or 2 described below is caused, resulting in low reliability on the analysis result, unless color forming sensitivity at the time of measurement is adjusted by some method.
1 When the sampling amount of the sample for analysis is determined so that the variation of the sampling amount may have substantially no influence on a measured value, the calibration range for analysis is narrowed unless the total amount of reagent solution used for the measurement is increased to some extent.
2 When the sampling amount of the sample is reduced for reducing the total amount of reagent solution used for the measurement, the within-run precision is lowered, and the influence of soiling of instruments used is enhanced as much.
Particularly when analysis for calcium or magnesium in serum is carried out by means of an automatic analyzer having restrictions such as the lower limit of the sampling amount of a sample and the upper limit of the sampling amount of reagent for analysis, it is very difficult to maintain a satisfactory within-run precision and a suitable calibration range, unless the color forming sensitivity at the time of measurement is adjusted by a suitable method.
As a method for adjusting the color forming sensitivity in such a measurement as described above, there are known, for example, a method of placing, in a reaction system, citric acid, tartaric acid or the like, which reacts with metal ions to form a colorless chelate, and a method using a carbonate buffer solution as a buffer solution for measurement. However, the former method is disadvantageous in that the reliability of measured values is lowered because the constant of chelate formation by citric acid or the like and the constant of chelate formation by a chelating reagent vary greatly with temperature. The latter method has, for example, the following disadvantages. The concentration of the buffer should be high for preventing the pH from being lowered by the absorption of carbon dioxide in the air, but when it is too high, the color forming sensitivity is greatly lowered, so that the measurement is hindered. Moreover, the buffer solution has a buffer capacity only in a narrow pH range, and therefore the kind of usable chelating reagent is limited.